In perhaps a nod to our old belief of the Earth as the center of the universe, our first unit of measurement is based on the distance of our home to the sun. We are 93 million miles from the sun, but it's much simpler to say we're one astronomical unit (AU) from the sun. In our solar system, the distance from the sun to the other planets can be measured in astronomical units as well: Mercury .38 AUs, Venus .72 AUs, Mars 1.52 AUs, Jupiter 5.2 AUs, Saturn 9.54 AUs, Uranus 19.218 AUs, Neptune 30.06 AUs, and Pluto 39.5 AUs.
This works great within our own solar system, but as we move through our galaxy, distances grow ever farther apart. That's why we created a unit of measure based on the distance that light travels in a year. We call these units "light years," of course. A light year is 6 trillion miles. (6,000,000,000,000 miles)
The closest star to our solar system is actually a system of three stars called the Alpha Centauri System, consisting of Alpha Centauri, Rigil Kentaurus, and Proxima Centauri, which is actually slightly closer than her sisters. Alpha Centauri is 4.3 light years from Earth.
If we want to move beyond our "neighborhood," our nearest neighboring galaxy is Andromeda. At 2.2 million light years, it's the most distant object that we can see without a telescope, and the only object outside our own galaxy in the universe.
2.2 million light years is a huge distance, but merely a drop in the bucket to the size of our universe. In order to measure larger distances, the parsec (Paralax Second) was invented. A parsec is approximately 3.258 light years. Along with the parsec, larger distances are measured in kiloparsecs (thousand parsecs) and megaparsecs (million parsecs).
One other way to denote very large numbers is something called scientific notation. This system is based on the number ten and is written like this 1 X 101. This number equals 10. The small 1 located to the right of the 10 indicates how many times 10 is used as a multiplier. In this case once, so the number equals 10. So, 1 X 102 would be the same as 1 X (10 X 10) or 100. An easy way to figure a scientific notation number out is to add the same number of zeros at the end as the small number to the right of 10. So, 1 X 105 would be 100,000. Small numbers can be written this way as well by using a negative power (the number to the right of 10). In that case, the number will tell you how many places to move the decimal point to the left. An example: 2 X 10-2 equals .02.